Known prior art panoramic dental X-rays machines are characterized by an arm rigidly interconnecting the tubehead and camera assemblies. The tubehead-camera assembly thus rotates as a unit around the head of the patient being radiographed. The patient is seated in the patient chair and positioned therein by means of a bite block slidably adjustably supported on a rod mounted in cantilever fashion, thus permitting undesirable head and jaw movement relative to the camera assembly notwithstanding the fact that the patient's chin is resting on a suitable chin rest assembly. The tubehead-camera assembly is then positioned to be in approximate visual alignment with some distinguishing feature of the patient's face. It is recognized that it is a most difficult task to accurately align the patient's head by conventional bite blocks, chin rests, and the like, since a dental arch varies in size and shape from person to person and bears no significant relationship to chin and head structure. Even when the same patient is radiographed on different occasions, adjustment and alignment are difficult to repeat.
It is noted that the distance from patient to film is much less than the distance from the patient to the tubehead. In a present model panoramic dental X-ray machine of the assignee, shown in FIG. 1 of the drawings, the distance between the film plane to teeth incisal edge is approximately 2.6" whereas the distance from the focal spot of the X-ray tube in the tubehead to teeth incisal edge is approximately 14.5". It is apparent that the patient to film distance is more critical and more sensitive to minor inaccuracies which can cause noticeable variations in the resultant image.
In the present invention, patient to film alignment and distance is fixed. Thus precise alignment of the tubehead to camera is less critical. It is appreciated that even though the camera assembly and tubehead assembly rotate as a unit in prior art machines, the camera is still vertically adjustable and must be aligned with the tubehead and patient. The patient's head, in the present invention, may be directly visually aligned with the film and camera.
The camera assembly, including the inner substantially semi-cylindrical drum or mask, are separated from the tubehead assembly and consequently do not rotate about the patient's head. The film is attached to and wrapped around a semi-cylindrical film carrier, although the invention is not limited to such configuration, and the film carrier caused to rotate at controlled non-uniform speeds in accordance with the type of radiograph desired. The film is thus limitedly rotated around the patient's face about a vertical axis parallel to the center line of the patient's face. A light-opaque but X-ray transmitting mask is interposed between the camera and patient, the mask rigidly supporting a bite block which is substantially inflexible.
The advantages of the present invention are many and varied. For example:
(a) Radiographing of the same or different patients are characterized by more repeatable positioning of both equipment and patients to provide more reliable radiographs resulting in improved diagnostic evaluations.
(b) Since the distance between the tubehead and camera may be varied, a more uniform and controlled magnification can be provided, not attainable with a fixed spacing therebetween.
(c) Human error is minimized as necessary manual adjustments by the operator are made easier.
(d) Weight of the rotating tubehead and camera assembly is markedly reduced. The sturdiness required in the structure illustrated in FIG. 1 of the drawings is necessitated by the horizontal cantilevered arm holding the camera. The entire "C" arm of this prior art structure can be eliminated and the tubehead mounted on a less massive pedestal or column to thus minimize mechanical complexity and clutter, as well as lowering cost, reducing vibrations, and facilitating radiographing in either direction of rotation of the tubehead assembly.
(e) Speed of rotation of the tubehead can be independently regulated to provide a wider focal trough at the centrals or incisors.
(f) The patient is not aware of camera rotation or movement about his face which may be psychologically more comforting.